This is the 9777th most frequent Polish word.
ziemią
"Ziemią" means "earth" or "soil" in the instrumental case in Polish.
Here, 'ziemią' is the instrumental form of 'ziemia' (earth), used to describe the entity with which the sky is meeting.
Niebo styka się z ziemią na horyzoncie.
The sky meets the earth at the horizon.
'Ziemią' here refers to soil, specifically the kind obtained from the forest, and denotes usage of this soil for gardening.
Uprawiam ogród ziemią z lasu.
I cultivate a garden with earth from the forest.
In this context, 'ziemią' is again the instrumental form of 'ziemia' indicating the medium used to cover the plants.